This invention relates to ion-exchange polymers useful in electrolytic cell membranes, as catalysts, and in membranes for the separation of miscible liquids and for the separation of gases.
Fluorinated polymers with pendent chains containing anionic groups such as sulfonamide and sulfonic acid are well known in the art. These polymers are known to be useful as ion-exchange polymers for use in electrolytic cells where thermal and chemical stability are required, such as membranes for use in an electrolytic chlor-alkali cell. In this type of application, the cell membrane must provide as low electric resistance as possible during the electrolysis, yet have superior selective ionic permeability.
In U.S. Pat. No. 4,000,057, the conditioning of a membrane suitable for use in an electrolytic cell is described. The membrane is expanded by immersing the membrane in a liquid solvation system in which the membrane exhibits a substantially flat expansion curve verses time for at least the first four hours after immersion. Suitable expansion solution components are water, ethylene glycol, and glycerin. Maximum expansion of the membrane is disclosed as about 3 percent using a 25 percent solution of glycerin in basic brine. The reference discloses that conditioning of a membrane by exposure to boiling water provides a water uptake for the membrane of about 50 to 60 percent in an 800 equivalent weight perfluorinated sulfonic acid group ion-exchange membrane and 20 to 28 percent by weight water uptake in a perfluorinated sulfonic acid group ion-exchange membrane having 1000 equivalent weight.
In U.S. Pat. No. 3,884,885, a fluorinated polymer containing pendent, ion-exchange groups in the form of --SO.sub.3 H or --SO.sub.2 NH.sub.2 is disclosed as being converted to a melt-processable polymer by reaction with a tertiary amine, quaternary ammonium base, or the salt of the amine or base having a molecular weight below 500. Treatment of the polymer with the amine or quaternary ammonium base by exposing a film of the polymer to the amine or quaternary ammonium base dissolved in a solvent results in a polymer containing a tertiary amine group or quaternary ammonium group attached to a portion of the polymer. In this form, the polymer can be processed or fabricated utilizing the application of heat. Prior to treatment with the amine or quaternary ammonium base, the polymer is intractable. Displacement of the tertiary amine group or quaternary ammonium group forming a part of the polymer is taught by treatment with a strong base such as an alkali metal hydroxide, which results in the replacement of the tertiary amine or quaternary ammonium groups with an alkali metal ion.
The preparation of other fluorocarbon cation-exchange polymer membranes is disclosed in U.S. Pat. No. 4,246,091 by reaction of a cation-exchange membrane carrying as its ion-exchange radical a sulfonic acid radical with primary to tertiary monoamines or, alternatively, with quaternary ammonium salts. Improved ionic selectability is disclosed for the treated fluorocarbon ion-exchange polymer membranes. Treatment of the fluoropolymer ion-exchange polymer membrane is accomplished by dipping the membrane into an aqueous solution of a monoamine salt at a temperature between room temperature and the boiling point of the aqueous monoamine salt solution. Subsequent to this treatment, a heat treatment step is required to prevent the improved ionic selectivity from dissipating within a relatively short time after treatment. The heat-treating temperature is, generally, higher than 100.degree. C. and is, preferably, higher than 140.degree. C. The reference discloses that improved ionic selective performance is demonstrated by increased current efficiency in comparison with similar untreated membranes when the membranes are utilized in an electrolytic cell for the electrolysis of sodium chloride aqueous solutions.
In U.S. Pat. No. 4,617,163, membrane sheets of a cation-exchange perfluorinated polymer containing sulfonic acid groups in the sodium salt form are reacted with an aqueous solution of a salt of a primary, secondary, or tertiary amine or a quaternary ammonium salt in aqueous solution prior to stretching the membrane sheet so as to increase the surface area per unit weight of the membrane. This permits the membrane to be secured in a stretched state as a membrane in an electrolytic cell so that during operation of the electrolytic cell, the previously stretched membrane remains taut and unwrinkled. The anionic groups in the fluoropolymer ion-exchange membrane can be derived from sulfonic acid, a carboxylic acid, or phosphonic acid. The membrane can be stretched subsequent to reaction with the primary, secondary, or tertiary amine or a quaternary ammonium salt. Surface area increases resulting from stretching the membrane are disclosed as being at least 5 percent to at least 50 percent per unit weight of the membrane. Replacement of the organic group residues of the amine salt or quaternary ammonium salt can be effected by treatment with an alkali metal hydroxide.
The separation of organic liquids utilizing an ion-exchange membrane is disclosed by Pasternak et al. in U.S. Pat. Nos. 4,798,674; 4,877,529; 4,952,318; and in U.S. Pat. No. 5,006,576. These references disclose the use of perfluorinated ion-exchange membranes which have been contacted with a quaternary ammonium salt in which the preferred organic group is a lower alkyl.
In U.S. Pat. No. 4,791,081, heterogeneous active catalysts are disclosed which are prepared by coating a carrier substrate with an aqueous emulsion containing a fluorocarbon sulfonic acid polymer. The substrate has a pore size of at least about 0.1 micrometer and the polymer has pendent chains containing sulfonyl groups which are subsequently converted to sulfonic acid or sulfonamide groups. Improved catalytic efficiency of the polymer is obtained by supporting the polymer on a carrier.
The coating of the polymer on a carrier has been found to increase accessibility of the reactive acid groups in the polymer and, therefore, results in an improvement in the catalytic reactivity of the polymer. Porous alumina and silicon carbide carriers have been used for this application and the resulting catalysts exhibit greatly increased reactivity. However, the catalytic activity of such catalysts may be less than desirable for certain applications.